Volume 18, Number 3 .... September 1996

Revisiting the RH Battlefield: Analysis of Risk and Cost

by Steven Weintraub

RH STANDARDS: The Past, The Present

In order to understand the context of the "Great RH Debate", it is important to understand the evolution of our currently accepted RH standards. Specialists have understood for a long time that RH stability was essential for the avoidance of damage induced by the dimensional change in organic materials. It is important to realize that the evolution of RH standards was based on practical considerations of what was achievable given the technology of the time, rather than what was essential for the preservation of any specific type of object or material (see J.P. Brown and W.B. Rose, "Origins of humidity recommendations and moisture control". APT Bulletin (1996) XXVII, #3).

In brief, the standards for acceptable RH in new museum construction in the U.S. became tighter through the 1960's and 1970's as a result of the increasing emphasis on conservation in museums. This was accompanied by a greater acceptance of the need for air conditioning in public spaces based on human comfort needs. In order to design a climate control system, the mechanical engineering team would ask the museum to provide a specification of maximum and minimum acceptable RH and temperature values. Since conservators lacked any guidelines on an exact RH and temperature value or range of fluctuations that would prevent damage, the decision was frequently made on the basis of the narrowest range of fluctuation that could be achieved by available equipment. This rationale was based on the assumption that there was a direct correlation between increased fluctuation and increased risk of damage. Over the years, technology improved to the point where engineers could design systems capable of producing "flat-line" environments (+/- 3 to 5%). While the cost of such a system was of concern to those handling finances, the conservator was rarely brought into such discussions, and was generally unaware of the cost implications of their recommendations.

Under the circumstances, it was reasonable for the conservator to ask for the tightest RH and temperature conditions possible, built around a "safe" set-point such as 45% or 50% RH, based on the assumption that less fluctuation resulted in less damage to collections. As the acceptable standard for RH narrowed for new museum construction, specialists tried to apply such standards to the retrofit of older buildings. However, such retrofits had to take into account the risk to the building envelope of holding a high interior winter RH. Also, it was more difficult to install such precision climate control equipment in older buildings, and the retrofit often resulted in a level of unacceptable damage to historically important structures. Finally, with the decreasing financial capability of cultural organizations, cost became more of a factor. As a result of such considerations, many conservators accepted the concept of a seasonally adjusted RH value. Typically, the RH set point might swing from 50 or 55% in the summer to 40 or 45% in the winter. Generally, conservators still wanted to maintain a minimum range of fluctuation at any one point in time such as +/- 5%, but such a seasonal swing would translate into a total annual acceptable RH range of 40-55%.


The research results of the Smithsonian Institution's Conservation Analytical Laboratory (CAL) along with other such research is important because it provides information on how museum objects actually respond to and get damaged by varying temperature and relative humidity conditions. Since the current controversy about appropriate RH standards was catalyzed by CAL's recent publications, the balance of this section will focus on the CAL results.

It is well known that many "museum objects" survive without apparent dimensionally induced damage in uncontrolled environments. For example, many canvas paintings remain in excellent condition although they have been kept in uncontrolled RH environments for centuries. It has also been our experience that some objects do get damaged in such environments. Unfortunately, we have very little data to allow us to relate such damage to specific RH conditions. Therefore, CAL undertook an extensive series of studies, described in the WAAC Newsletter (Jan. 1995, Vol.17, No.1) and elsewhere. Based on their results, they determined that virtually all museum objects composed of organic materials can sustain an infinite range of fluctuation within the range of 35-65% RH without damage.

Most conservators would agree that it is not necessary to maintain all organic materials in a very tight RH condition such as 50-55% RH. However, many conservators remain skeptical about the recommendation of 35-65% RH for two reasons.

  1. Actuarial Risk: Although it is probably true that many objects can sustain fluctuation in the range of 35-65% RH without physical damage, what are the exceptions? Do the exceptions constitute 1% of the collection, 10% or 0.1%. These are important numbers to determine. For example, many conservators can cite practical experiences of problems with illuminated vellums or French Marquetry in fluctuating RH conditions that were narrower than CAL's 35-65% range. Unfortunately, it is hard to document such cases with specific RH histories and damage, but it is difficult to ignore such experiences. Although most conservators would agree that there is virtually no risk of dimensionally induced damage within a very narrow range such as 50-55%, or even within a somewhat expanded range such as 45-55%, the level of concern increases as the range and degree of fluctuation increases beyond a range of 10-15%. Conservators are not ignoring the CAL research. However, most conservators do not agree that all organic materials are safe when subjected to large short and long term fluctuations in the range of 35-65% RH. The exceptions must be clearly identified and understood. In essence, most conservators believe that there is not a magic safe RH range for all objects. Rather, there is a question of relative risk. The narrower the RH range, and the less frequency and size of RH fluctuations, the less risk for the object. Although the CAL research suggests that there is less of a risk to many categories of museum collections than previously thought, the level of risk must still be defined. And, for the moment, most conservators feel that such a level of risk has not been well enough defined for all materials in museum collections.

  2. Actual RH Conditions: Another concern is how the range of 35-65% RH will be interpreted by museum administrators, facilities staff, and engineers. It has been the experience of most conservators that a specification of 35-65% RH will result in conditions that sometimes extend beyond the specified range. Therefore, a specification such as 40-60% would probably be necessary to assure conditions within the wider CAL recommendations. Again, it is necessary to consider risk, in this case risk of faulty system operation, in terms of safety margins for collections.

Unfortunately, space does not allow a fuller discussion of actuarial risk to collections and practical experience with engineering fidelity to humidity specifications. In summary, the key point is that the conservator must evaluate relative risk to collections and must have a sense of what it means to maintain a collection in an environment of +/- 5%, 10% or 15%, both in terms of probable risk of damage, and cost for maintaining such conditions. If, for example, the risk is very low at 45-55% RH and the cost of installing and operating a mechanical system is very high, is such a cost justifiable? At what point does a certain level of risk justify a particular system cost. There is no black and white answer, and most conservators believe that the recommendation of 35-65% RH is too simplistic to help them arrive at a reasonable and responsible RH recommendation.

CLIMATE CONTROL: Options and Costs

The fundamental issue of climate control comes down to system costs versus risk to collections and buildings. It is essential to weigh all three issues. The greatest weakness in the recent round of CAL publications is with regard to system costs. It is absolutely true that it is more expensive to maintain tighter RH control, both in terms of system installation and system operation. However, it is not fair to compare a "flat-line" environment to a 35-65% RH environment, as if there is no middle ground. Flat-line environments are very expensive to install and maintain because they require complex multi-zone equipment, and must operate in an energy inefficient manner. If the engineer interprets a 35-65% RH specification as a justification for installing minimum humidification, no re-heat capacity and a simple, inexpensive system of controls, there is a large financial savings over "flat-line" systems. But this should not be the question. I am very uncomfortable with the notion of installing any air conditioning system in a museum without some level of dehumidification control, and this type of control requires both re-heat capacity and proper controls. Similarly, I am uncomfortable with the idea of installing minimum humidification in a museum (limited to 35% RH) when it does not cost much more to install somewhat higher capacity systems. It is better to install somewhat higher capacity systems and choose to operate them at a lower capacity if energy savings, or building envelope issues are a concern. Finally, I consider it unwise to install a new environmental system with a simple control system that cannot provide the possibility of tight control over the HVAC equipment.

As a conservator, I want to have the option of looking at the cost of a system that can maintain +/- 5% or 10% or 15% RH. I want to look at the risk or cost of modifying the building envelope to accommodate such systems. Where is the acceptable break-point in terms of system cost and performance. I would like to know how much I save by installing and operating a system at 40-55% versus 35-65%. It is important for the HVAC engineer to explain at what point and why costs may jump, depending on a particular RH specification. In this way, the conservator could specify an RH range based on a knowledge of the cost implication of such a recommendation.


In the final analysis, the conservator and other concerned parties must look at the relative risk to collections for various ranges of RH and the cost for achieving such a range. Although the CAL research confirms practical experience that not all things get damaged in a range outside 45-55% RH, there are still many reservations about the relative risk of maintaining a system at 35-65% RH. Until we are more comfortable with a full knowledge of humidity-induced damage, the conservator should be allowed to consider various options for a narrower RH range that are justifiable, based on an understanding of system costs and collection risks. In this regard, further work such as CAL's investigations on humidity response of materials should be encouraged. However, the final interpretation and use of this information should be left to the individual conservator.

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